The invention relates to a synthesis method for carbenicillindisodium. The carbenicillindisodium is prepared sequentially through the following steps: adding 6-amino penicillanic acid and a silanized agent into ethyl acetate, and after stirring, adding 2,2-dimethyl-5-phenyl-1,3-dioxane-4,6-diketone till the reaction is completed; adding water and alkaline into the reaction mixture, and regulating the pH value to between 5.0 and 8.5 percent; after carrying out the phase separation, collecting the water phase, adding ethyl acetate, regulating the pH value to between 1.0 and 4.0 through acid, carrying out the phase separation again, and collecting the organic phase; and adding ethyl acetate and active carbon into the organic phase for stirring and filtration, adding sodium iso-octoate solution into the filtrate, filtrating out the precipitate, cleaning the precipitate through acetone, and carrying out the vacuum drying of the precipitate to obtain the carbenicillindisodium. The synthesis method has easily obtained and cheap raw materials, moderate reaction conditions and high yield, and is suitable for the industrialized production. The solvents used are only water and ethyl acetate, thereby reducing the harm of the solvents on operating personnel, and bringing about the convenient reclamation and treatment of the solvents.